Modification of the Palmer Drought Severity Index (PDSI) based on Atmosphere-Land Surface Interaction Scheme (ALSIS) in Karkheh River Basin

In this study, the water balance component of the Palmer Drought Severity Index (PDSI) was replaced by the Atmosphere-Land Surface Interaction Scheme (ALSIS) and the modified Palmer Drought Severity Index (AL-PDSI) was compared with the self-calibrating version of Palmer Drought Severity Index (SC-PDSI). The evaluation of drought indices in Karkheh river basin for the period 1983-2011 reveals reporting more severe droughts for certain duration by the AL-PDSI in comparison with the SC-PDSI. The AL-PDSI can capture seasonal variations in relative frequency of droughts. Moreover, the relative frequency of mild, moderate and severe drought for the AL-PDSI is higher than that for the SC-PDSI. However, the AL-PDSI and SC-PDSI do not show a remarkable difference in terms of the correlation with vegetation and soil moisture variations. The more physical mechanism of AL-PDSI helps in more comprehensive understanding of drought characteristics and in studying the effects of climate and land use change scenarios on droughts.